By the way, after speaking with several researchers in aquatic botany, I've been unable to find critical in situ CO2 measurement like the readings I got from the CO2 meter probe.
No one has really ever done a good study using it in natural systems.
Seems like they'd want to measure the most typically limiting component of submersed plant growth.
Give what we know about morphology, flow and CO2 differences in our tanks, a simple issue like differences in CO2 uptake between species would be a strong reason for one of those species not to do well together. Many claim allelopathic reasons. I've never been able to show this even with 300 + species. Some critics have claimed perhaps I just needed higher biomass of one or the other species, but this does not seem like that valid of an argument.
If it where, typically with allelopathic chemicals, we see a a strong response over a wide range of concentrations of that chemical on the susceptible plants. In otherwords, the concentration is less important.
Still, some plants are far more aggressive at CO2 uptake than others.
This can be measured and has been. Given the large role CO2 plays and at limiting levels, this can really have a dramatic effect.
It seems that this issue, rather than allelopathy, whether it is plant=plant or algae - plant is really the main driver.
To test this, we can (and many have), manipulated CO2 and have had repeatable and strong responses from plants. Some die etc, most get algae.
With allelopathy, we can test using ACTIVATED carbon to remove any alleopathic chemicals, again, many have done this and no one has EVER reported a correlation between algae and using activated carbon that I or others are aware of.
So given the observations and simple test a hobbyist can do and draw from, this entire idea, while perhaps a sexy idea, is pretty weak in it's logic, experimental results, and the research which shows little(no) support to date in any natural system.
Questions arise about how to tease apart the other causes to poor plant growth, algae blooms etc, these must be addressed and ruled out before being left with some pie in the sky theory.
Little by little, test by test, year to year, you pick away at the questions and get around to testing it and seeing.
Somethings might require $$ equipment, some can be explained well using basic hobbyists planted tanks and test.
Regards,
Tom Barr